The present invention relates to a system of heating elements of the type comprising resistors connected in series in the form of linear plates, more particularly intended for the construction of thermal printing heads. It also relates to the control circuit for the thermal printing heads, which is simplified by the adopting of resistors according to the invention.
Thermal printers are peripheral devices in information processing or telecommunications systems in which the printing of a line of text is obtained by means of a strip of heating resistors. The heat given off by an elementary resistor chemically modifies the paper on which printing is taking place. The printing of a line of characters by means of a thermal printing head is obtained by the repetition of several lines of dots at the rate of 8 dots per millimeter. A thermal printing head for a standard paper size of width 21 cm comprises 1728 resistors deposited on a glass or ceramic plate. The width of each resistor is approximately 250 microns and they are also 250 microns apart.
There are two problems in connection with thermal printing heads, namely that of the control of a given resistor and that of heat dissipation. Each unitary programmed resistor is controlled by a circuit comprising, inter alia, two transistors and a diode. The diodes which are connected in series with the non-programmed resistors limit the potential at the terminals thereof and prevent heating thereof. Thus, a thermal printing head requires a circuit having the same number of diodes as there are heating resistors or preferably, as a function of the diagram adopted, a number of diodes which is equal to half the number of heating resistors, there being a large number of diodes because there are at least 863 diodes for 1728 points.